Radiation imaging apparatus, radiation imaging system, and radiation imaging method

ABSTRACT

A radiation imaging apparatus that obtains a radiation image by an energy subtraction method. Each pixel includes a conversion element that converts radiation into an electrical signal and a reset portion that resets the conversion element. Each pixel performs an operation of outputting a first signal corresponding to an electrical signal generated by the conversion element in a first period, and an operation of outputting a second signal corresponding to an electrical signal generated by the conversion element in the first period and a second period. Radiation having first energy is emitted in the first period, and radiation having second energy is emitted in the second period. In each pixel, the reset portion does not reset the conversion element during a period that includes the first period and the second period.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a radiation imaging apparatus, aradiation imaging system, and a radiation imaging method.

Description of the Related Art

There is an energy subtraction method as an imaging method that appliesa radiation imaging apparatus. The energy subtraction method is a methodof obtaining new images (for example, a bone image and a soft tissueimage) by processing a plurality of images obtained by capturing anobject a plurality of times while changing energy of radiation toirradiate the object. A time interval during which a plurality ofradiation images are captured is, for example, several seconds or morein a radiation imaging apparatus to capture a still image, about 100msec in a general radiation imaging apparatus for a moving image, andabout 10 msec even in a radiation imaging apparatus for a high-speedmoving image. If the object moves in this time interval, an artifact iscaused by that movement. It is therefore difficult to obtain, by theenergy subtraction method, a radiation image of an object such as aheart that moves fast.

Japanese Patent Laid-Open No. 2009-504221 describes a system thatperforms dual energy imaging. In this system, the tube voltage of anX-ray source is set to the first kV value, and then changed to thesecond kV value in imaging. Then, the first signal corresponding to thefirst sub-image is integrated when the tube voltage is the first kVvalue, and integration is reset after the integrated signal istransferred to a sample and hold node. Subsequently, the second signalcorresponding to the second sub-image is integrated when the tubevoltage is the second kV value. Consequently, readout of the integratedfirst signal and integration of the second signal are performedparallelly.

A method described in Japanese Patent Laid-Open No. 2009-504221 performsreadout of the integrated first signal and integration of the secondsignal parallelly, making it possible to shorten a time interval duringwhich two images for the energy subtraction method are captured. In themethod described in Japanese Patent Laid-Open No. 2009-504221, however,a reset operation exists after integration and transfer of the firstsignal corresponding to the first sub-image in order to obtain tworadiation images (the first sub-image and the second sub-image). When aradiation irradiation time is shortened up to about 1 msec in order tosuppress the influence of an object movement, the object is irradiatedwith radiation wastefully for a time at 10 percent of the radiationirradiation time even if the reset operation can be completed in 0.1msec.

SUMMARY OF THE INVENTION

The present invention provides a technique advantageous in obtaining aradiation image for an energy subtraction method in a shorter time whilereducing radiation irradiation that does not contribute to imaging.

One of the aspects of the present invention provides a radiation imagingapparatus that obtains a radiation image by an energy subtraction methodof obtaining a new image by processing a plurality of images obtained bycapturing an object a plurality of times while changing energy ofradiation to irradiate the object, the apparatus comprising: a pixelarray that includes a plurality of pixels, wherein each of the pluralityof pixels includes a conversion element that converts radiation into anelectrical signal and a reset portion that resets the conversionelement, each of the plurality of pixels performs an operation ofoutputting a first signal corresponding to an electrical signalgenerated by the conversion element in a first period, and an operationof outputting a second signal corresponding to an electrical signalgenerated by the conversion element in the first period and a secondperiod different from the first period, radiation having first energy isemitted in the first period, and radiation having second energy isemitted in the second period, and the radiation imaging apparatus has amode in which, in each of the plurality of pixels, the reset portiondoes not reset the conversion element during a period that includes thefirst period and the second period.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the arrangement a radiation imaging systemaccording to an embodiment of the present invention;

FIG. 2 is a view showing an example of the arrangement of a radiationimaging apparatus;

FIG. 3 is a circuit diagram showing an example of the arrangement of apixel;

FIG. 4 is a circuit diagram showing another example of the arrangementof the pixel;

FIG. 5 is a timing chart showing an operation in the first mode;

FIG. 6 is a timing chart showing an operation in the second mode;

FIG. 7 is a timing chart showing an operation in the third mode;

FIG. 8 is a timing chart showing an operation in the fourth mode;

FIG. 9 is a timing chart showing an operation in the fifth mode; and

FIG. 10 is a timing chart showing an operation in the sixth mode.

DESCRIPTION OF THE EMBODIMENTS

An exemplary embodiment of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 shows the arrangement of a radiation imaging system 1 accordingto an embodiment of the present invention. The radiation imaging system1 includes a radiation imaging apparatus 100. The radiation imagingsystem 1 or the radiation imaging apparatus 100 is a system or apparatusfor obtaining a radiation image by an energy subtraction method. Theenergy subtraction method is a method of obtaining new images (forexample, a bone image and a soft tissue image) by processing a pluralityof images obtained by capturing an object a plurality of times whilechanging energy of radiation to irradiate the object. The term radiationcan include, for example, α-rays, β-rays, γ-rays, particle rays, andcosmic rays in addition to X-rays.

The radiation imaging system 1 can include a radiation source 400 thatgenerates radiation, an exposure control apparatus 300 that controls theradiation source 400, and a control apparatus 350 that controls theexposure control apparatus 300 (radiation source 400) and the radiationimaging apparatus 100. The control apparatus 350 can include a signalprocessor 352 that processes a signal supplied from the radiationimaging apparatus 100. All or some functions of the control apparatus350 can be incorporated in the radiation imaging apparatus 100.Alternatively, some functions of the radiation imaging apparatus 100 canbe incorporated in the control apparatus 350. The control apparatus 350can be formed by a computer (processor) and a memory that storesprograms provided for the computer. The signal processor 352 can be madeof some of the programs. Alternatively, the signal processor 352 can bemode of a computer (processor) and a memory that stores programsprovided for the computer. The control apparatus 350 may be formed by aDSP (digital signal processor) or a PLA (programmable logic array)entirely or partially. The control apparatus 350 and the signalprocessor 352 may be designed and manufactured by a logic synthesis toolbased on a file that describes their operations.

The exposure control apparatus 300 can include, for example, an exposureswitch and in response to the fact that the exposure switch is turnedon, cause the radiation source 400 to emit radiation and notify thecontrol apparatus 350 of information indicating a timing at which theradiation is emitted. Alternatively, the exposure control apparatus 300causes the radiation source 400 to emit radiation in accordance with acommand from the control apparatus 350.

The radiation source 400 has a function of changing radiation energy(wavelength). The radiation source 400 can change the radiation energyby, for example, changing a tube voltage (a voltage applied between thecathode and anode of the radiation source 400). The radiation source 400can emit radiation having a plurality of different kinds of energies.

The radiation imaging apparatus 100 includes a pixel array 110 thatincludes a plurality of pixels. Each of the plurality of pixels includesa convertor that converts radiation into an electrical signal (forexample, charges) and a reset portion that resets the convertor. Eachpixel may be configured to convert the radiation into the electricalsignal directly or may be configured to convert the radiation into lightsuch as visible light, and then convert the light into the electricalsignal. In the latter case, a scintillator for converting radiation intolight can be used. The plurality of pixels that form the pixel array 110can share the scintillator.

FIG. 2 shows an example of the arrangement of the radiation imagingapparatus 100. As described above, the radiation imaging apparatus 100includes the pixel array 110 that includes a plurality of pixels 112.The plurality of pixels 112 can be arrayed so as to form a plurality ofrows and a plurality of columns. The radiation imaging apparatus 100 canadditionally include a row selection circuit 120 that selects the rowsof the pixel array 110. The row selection circuit 120 selects the rowsby driving row control signals 122.

The radiation imaging apparatus 100 can also include a readout circuit140 that reads out signals from the pixels 112 of the row selected bythe row selection circuit 120 out of the plurality of rows of the pixelarray 110. The readout circuit 140 reads out signals for the pluralityof columns output to a plurality of column signal transmission paths 114of the pixel array 110. The column signal transmission path 114 of eachcolumn can include, for example, a plurality of column signal lines thattransmit a plurality of signals detected by the pixels 112. For example,the noise levels of the pixels 112 and radiation signals correspondingto radiation detected by the pixels 112 can be output to the pluralityof column signal lines. The readout circuit 140 can be configured toread out the noise levels and the radiation signals, respectively,output to the column signal transmission paths 114.

The radiation imaging apparatus 100 can include a column selectioncircuit 150 that selects, in a predetermined order, signals for theplurality of columns read out from the pixels of the rows of the pixelarray 110 selected by the readout circuit 140. The radiation imagingapparatus 100 can also include an amplifier unit 160 that amplifies thesignals selected by the column selection circuit 150. Note that when thereadout circuit 140 reads out a pair of the noise level and radiationsignal from each pixel 112, the amplifier unit 160 may be configured asa differential amplifier that amplifies a difference between theradiation signal and the noise level forming the pair or may beconfigured to amplify them individually. The radiation imaging apparatus100 can further include an A/D convertor 170 that A/D-converts a signalOUT output from the amplifier unit 160 and outputs a digital signal DOUT(radiation image signal).

The radiation imaging apparatus 100 can include a timing generator (canalso be referred to as a controller or a state machine) 130 thatcontrols the row selection circuit 120, the readout circuit 140, thecolumn selection circuit 150, and the amplifier unit 160.

FIG. 3 shows an example of the arrangement of one pixel 112. The pixel112 includes, for example, a conversion element 210, a reset switch 220(reset portion), an amplifier circuit 230, a sensitivity changingportion 240, a clamp circuit 260, sample and hold circuits (holdingportions) 270, 280, and 290, and an output circuit 310.

The conversion element 210 converts radiation into an electrical signal.The conversion element 210 can be formed by, for example, a scintillatorthat can be shared by the plurality of pixels and a photoelectricconversion element. The conversion element 210 includes a chargeaccumulation portion that accumulates a converted electrical signal(charges), that is, an electrical signal corresponding to radiation. Thecharge accumulation portion is connected to the input terminal of theamplifier circuit 230.

The amplifier circuit 230 can include MOS transistors 235 and 236, and acurrent source 237. The MOS transistor 235 is connected to the currentsource 237 via the MOS transistor 236. The MOS transistor 235 and thecurrent source 237 form a source follower circuit. The MOS transistor236 is an enable switch which is turned on by activating an enablesignal EN, and sets the source follower circuit formed by the MOStransistor 235 and the current source 237 in an operation state.

The charge accumulation portion of the conversion element 210 and thegate of the MOS transistor 235 function as a charge-voltage convertorCVC that converts charges accumulated in the charge accumulation portioninto a voltage. That is, a voltage V (=Q/C) determined by charges Qaccumulated in the charge accumulation portion and a capacitance value Cof the charge-voltage convertor appears in the charge-voltage convertorCVC. The charge-voltage convertor CVC is connected to a reset potentialVres via the reset switch 220. When a reset signal PRES is activated,the reset switch 220 is turned on, and the potential of thecharge-voltage convertor is reset to the reset potential Vres. The resetswitch 220 can include a transistor that has the first main electrode(drain) connected to the charge accumulation portion of the conversionelement 210, the second main electrode (source) to which the resetpotential Vres is applied, and a control electrode (gate). Thetransistor electrically connects the first main electrode and the secondmain electrode by receiving an ON voltage at the control electrode, andresets the charge accumulation portion of the conversion element 210.

The clamp circuit 260 clamps, by a clamp capacitor 261, a reset noiselevel output from the amplifier circuit 230 in accordance with thepotential of the reset charge-voltage convertor CVC. The clamp circuit260 is a circuit configured to cancel the reset noise level from asignal (radiation signal) output from the amplifier circuit 230 inaccordance with charges (electrical signal) converted by the conversionelement 210. The reset noise level includes kTC noise at the time ofreset of the charge-voltage convertor CVC. A clamp operation isperformed by turning on a MOS transistor 262 by activating a clampsignal PCL, and then turning off the MOS transistor 262 by deactivatingthe clamp signal PCL.

The output side of the clamp capacitor 261 is connected to the gate of aMOS transistor 263. The source of the MOS transistor 263 is connected toa current source 265 via a MOS transistor 264. The MOS transistor 263and the current source 265 form a source follower circuit. The MOStransistor 264 is an enable switch which is turned on by activating anenable signal ENO supplied to its gate, and sets the source followercircuit formed by the MOS transistor 263 and the current source 265 inan operation state.

The output circuit 310 includes MOS transistors 311, 313, and 315 androw selection switches 312, 314, and 316. The MOS transistors 311, 313,and 315, respectively, form source follower circuits with currentsources (not shown) connected to column signal lines 321, 322, and 323.

The sample and hold circuit 280 (the first holding portion or the firstsignal holding portion) can sample and hold (hold) a radiation signal(first signal) as a signal output from the clamp circuit 260 inaccordance with charges generated in the conversion element 210. Thesample and hold circuit 280 can include a switch 281 and a capacitor282. The switch 281 is turned on by activating a sample and hold signalTS1. The radiation signal (first signal) output from the clamp circuit260 is written in the capacitor 282 via the switch 281 by activating thesample and hold signal TS1.

In the example shown in FIG. 3, the pixel 112 can include the additionalsample and hold circuit 290 (second holding portion) configured to writea radiation signal. The sample and hold circuit 290 can sample and hold(hold) a radiation signal (second signal) as a signal output from theclamp circuit 260 in accordance with charges generated in the conversionelement 210. The sample and hold circuit 290 can include a switch 291and a capacitor 292. The switch 291 is turned on by activating a sampleand hold signal TS2. The radiation signal (second signal) output fromthe clamp circuit 260 is written in the capacitor 292 via the switch 291by activating the sample and hold signal TS2. The pixel 112 may furtherinclude an additional sample and hold circuit configured to write aradiation signal. That is, the pixel 112 can include a plurality (thearbitrary number) of sample and hold circuits (holding portions)configured to write radiation signals.

In a state in which the reset switch 220 resets the potential of thecharge-voltage convertor CVC, and the MOS transistor 262 is turned on,the clamp circuit 260 outputs the noise level (offset component) of theclamp circuit 260. The sample and hold circuit 270 (second signalholding portion) can sample and hold (hold) the noise level of the clampcircuit 260. The sample and hold circuit 270 can include a switch 271and a capacitor 272. The switch 271 is turned on by activating a sampleand hold signal TN. A noise level output from the clamp circuit 260 iswritten in the capacitor 272 via the switch 271 by activating the sampleand hold signal TN. In this embodiment, the sample and hold circuit 270(second signal holding portion) can also be used to hold a radiationsignal as a signal output from the clamp circuit 260 in accordance withcharges generated in the conversion element 210.

When row selection signals VST are activated, signals corresponding tosignals held by the sample and hold circuits 270, 280, and 290 areoutput to the column signal lines 321, 322, and 323 that form the columnsignal transmission paths 114. More specifically, a signal Ncorresponding to a signal (a noise level or a radiation signal) held bythe sample and hold circuit 270 is output to the column signal line 321via the MOS transistor 311 and the row selection switch 312. A signal S1corresponding to a signal (first radiation signal) held by the sampleand hold circuit 280 is output to the column signal line 322 via the MOStransistor 313 and the row selection switch 314. A signal S2corresponding to a signal (second radiation signal) held by the sampleand hold circuit 290 is output to the column signal line 323 via the MOStransistor 315 and the row selection switch 316.

The pixel 112 may include addition switches 301, 302, and 303 configuredto add signals of the plurality of pixels 112. In an addition mode,addition mode signals ADDN, ADDS1, and ADDS2 are activated. Thecapacitors 272 of the plurality of pixels 112 are connected to eachother by activating the addition mode signal ADDN, averaging signals(noise levels). The capacitors 282 of the plurality of pixels 112 areconnected to each other by activating the addition mode signal ADDS1,averaging signals. The capacitors 292 of the plurality of pixels 112 areconnected to each other by activating the addition mode signal ADDS2,averaging signals.

The pixel 112 can include the sensitivity changing portion 240. Thesensitivity changing portion 240 can include switches 241 and 242,capacitors 243 and 244, and MOS transistors 245 and 246. When a firstchange signal WIDE is activated, the switch 241 is turned on, and thecapacitance value of the first additional capacitor 243 is added to thecapacitance value of the charge-voltage convertor CVC. Consequently, thesensitivity of the pixel 112 is decreased. Further, when a second changesignal WIDE 2 is also activated, the switch 242 is also turned on, andthe capacitance value of the second additional capacitor 244 is added tothe capacitance value of the charge-voltage convertor CVC. Consequently,the sensitivity of the pixel 112 is further decreased. A dynamic rangecan be widened by adding a function of decreasing the sensitivity of thepixel 112. An enable signal ENW may be activated when the first changesignal WIDE is activated. In this case, the MOS transistor 246 performsa source follower operation. Note that when the switch 241 of thesensitivity changing portion 240 is turned on, the potential of thecharge accumulation portion of the conversion element 210 may be changedby charge redistribution. Consequently, some signals may be destructed.

The above-described reset signal Pres, enable signal EN, clamp signalPCL, enable signal ENO, sample and hold signals TN, TS1, and TS2, androw selection signals VST are control signals controlled by the rowselection circuit 120 and correspond to the row control signals 122 ofFIG. 2.

FIG. 4 shows another example of the arrangement of the pixel 112. In theexample shown in FIG. 4, the pixel 112 includes the conversion element210, a switch 420, a reset switch 430, a capacitor 440, a MOS transistor450, a current source 460, and a row selection switch 470. Theconversion element 210 can have the same arrangement as theaforementioned conversion element 210. The switch 420 writes (that is,samples and holds), in the capacitor 440, charges accumulated in thecharge accumulation portion of the conversion element 210 by activatinga sample and hold signal TS driven by the row selection circuit 120. TheMOS transistor 450 forms a source follower circuit with the currentsource 460. The row selection switch 470 activates the row selectionsignals VST driven by the row selection circuit 120. When the rowselection switch 470 is turned on, the MOS transistor 450 outputs, tothe column signal transmission path 114, a signal corresponding to asignal held by the capacitor 440. Note that in the arrangement shown inFIG. 4, the potential of the charge accumulation portion of theconversion element 210 may be changed by charge injection when theswitch 420 is turned on. Consequently, some signals may be destructed.

On the other hand, in the pixel 112 having the arrangement as shown inFIG. 3, signals are not destructed in, for example, the chargeaccumulation portion of the conversion element 210 in a sample and holdoperation. That is, in the pixel 112 having the arrangement as shown inFIG. 3, the radiation signals can be nondestructively read out. Such anarrangement is advantageous to radiation imaging to which the energysubtraction method is applied to be described below and is particularlyadvantageous to the third to sixth modes to be described below.Therefore, an example will be described below in which the pixel 112 hasthe arrangement shown in FIG. 3.

The radiation imaging apparatus 100 and a radiation imaging method usingthis of this embodiment can have a plurality of modes for obtainingradiation images by the energy subtraction method. These modes will bedescribed below.

FIG. 5 shows the operation of the radiation imaging apparatus 100 orradiation imaging system 1 in the first mode. In FIG. 5, the abscissaindicates a time. “Radiation energy” is energy of radiation which isemitted from the radiation source 400 and irradiates the radiationimaging apparatus 100. “PRES” is the reset signal PRES. “DOUT” is anoutput of the A/D convertor 170. The control apparatus 350 can controlsynchronization of radiation emission from the radiation source 400 andthe operation of the radiation imaging apparatus 100. The timinggenerator 130 controls an operation in the radiation imaging apparatus100. The clamp signal PCL is also activated over a predetermined periodin a period during which the reset signal PRES is activated, and theclamp circuit 260 clamps a noise level.

The conversion element 210 is reset by activating the reset signal PRESover a predetermined period, and then radiation 501 having the firstenergy is emitted. Subsequently, charges (electrical signal) accumulatedin each pixel 112 of the pixel array 110 by the radiation 501 are outputas radiation signals 503 from the radiation imaging apparatus 100.

Subsequently, the conversion element 210 is reset by activating thereset signal PRES over the predetermined period, and then the radiation501 having the second energy different from the first energy is emitted.Subsequently, charges (electrical signal) accumulated in each pixel 112of the pixel array 110 by irradiation with radiation 502 are output fromthe radiation imaging apparatus 100 as a radiation signal 504. Thesignal processor 352 of the control apparatus 350 obtains a subtractionimage by processing the radiation signals 503 and 504 in accordance withthe energy subtraction method.

In the first mode, reset, irradiation with the radiation 501 of thefirst energy, output of the radiation signals 503 corresponding to it,reset, irradiation with the radiation 502 of the second energy, andoutput of the radiation signal 504 corresponding to it are performedsequentially. Therefore, the first mode is disadvantageous to radiationimaging of fast-moving object but is advantageous in capturing a stillobject accurately because it can obtain a radiation image of the firstenergy and a radiation image of the second energy while separating themcompletely.

Note that various methods can be adopted as the energy subtractionmethod. For example, it is possible, by calculating a difference betweenthe radiation image of the first energy and the radiation image of thesecond energy, to obtain a bone image and a soft tissue image. The boneimage and the soft tissue image may be generated by solving nonlinearsimultaneous equations based on the radiation image of the first energyand the radiation image of the second energy. It is also possible toobtain a contrast medium image and the soft tissue image based on theradiation image of the first energy and the radiation image of thesecond energy. It is also possible to obtain an electron density imageand an effective atomic number image based on the radiation image of thefirst energy and the radiation image of the second energy.

FIG. 6 shows the operation of the radiation imaging apparatus 100 orradiation imaging system 1 in the second mode. In FIG. 6, the abscissaindicates a time. “Radiation energy” is energy of radiation which isemitted from the radiation source 400 and irradiates the radiationimaging apparatus 100. “PRES” is the reset signal PRES. “TS1” is thesample and hold signal TS1. “DOUT” is an output of the A/D convertor170. The control apparatus 350 can control synchronization of radiationemission from the radiation source 400 and the operation of theradiation imaging apparatus 100. The timing generator 130 controls anoperation in the radiation imaging apparatus 100. The clamp signal PCLis also activated over a predetermined period in a period during whichthe reset signal PRES is activated, and the clamp circuit 260 clamps anoise level.

The conversion element 210 is reset by activating the reset signal PRESover a predetermined period, and then radiation 511 having the firstenergy and radiation 512 having the second energy different from thefirst energy are emitted. Note that the radiation 511 and the radiation512 may be emitted successively in terms of time or with a time intervalbetween them.

The irradiation times of the radiations 511 and 512 are preset, and asample and hold operation performed by the sample and hold circuit 280in accordance with the sample and hold signal TS1 ends immediatelybefore irradiation with the radiation 512. Subsequently, the conversionelement 210 is reset by activating the reset signal PRES over thepredetermined period. Note that in accordance with the sample and holdsignal TS1, the sample and hold circuit 280 samples and holds aradiation signal generated by irradiation with the radiation 511 havingthe first energy.

Signals sampled and held by the sample and hold circuit 280 inaccordance with the sample and hold signal TS1 are output as radiationsignals 513 from the radiation imaging apparatus 100. Subsequently, thesample and hold circuit 280 performs the sample and hold operation inaccordance with the sample and hold signal TS1. Consequently, the sampleand hold circuit 280 samples and holds radiation signals generated byirradiation with the radiation 512 having the second energy. Signalssampled and held by the sample and hold circuit 280 in accordance withthe sample and hold signal TS1 are output as radiation signals 514 fromthe radiation imaging apparatus 100. The signal processor 352 of thecontrol apparatus 350 obtains a subtraction image by processing theradiation signals 513 and the radiation signals 514 in accordance withthe energy subtraction method.

In the second mode, irradiation with the radiation 512 of the secondenergy is started before the output of the radiation signals 513 ends.Therefore, the second mode is superior to the first mode in radiationimaging of the fast-moving object. However, reset is performed in aperiod that includes a period during which the radiation 511 of thefirst energy is emitted and a period during which the radiation 512 ofthe second energy is emitted. Thus, information on radiation emitted ina reset period is lost by that reset. Consequently, image quality maydeteriorate accordingly.

The third to sixth modes to be described below are superior to the firstmode and the second mode in radiation imaging of the fast-moving object.In the third to sixth modes, each pixel 112 performs an operation ofoutputting the first signal corresponding to an electrical signalgenerated by the conversion element 210 in a first period T1 and anoperation of outputting the second signal corresponding to an electricalsignal generated by the conversion element 210 in a second period T2.Note that the second period T2 is different from the first period T1.Radiation having the first energy is emitted in the first period T1, andradiation having the second energy is emitted in the second period T2.In each of the plurality of pixels 112, during the period that includesthe first period T1 and the second period T2, the reset switch 220(reset portion) does not reset the conversion element 210 (the resetsignal Pres (voltage thereof) does not change). Hence, radiationinformation is never lost by reset during the period that includes thefirst period T1 and the second period T2. This is advantageous inobtaining a more accurate radiation image by the energy subtractionmethod while reducing wasteful radiation irradiation.

Note that in a case in which radiation having the third energy isemitted in addition to the radiation having the first and secondenergies, a third period T3 can be provided in addition to the firstperiod T1 and the second period T2, and the radiation having the thirdenergy can be emitted in the third period. In this case, in each of theplurality of pixels 112, during a period that includes the first periodT1, the second period T2, and the third period, the reset switch 220(reset portion) does not reset the conversion element 210. The first tothird energies can be different from each other. It is only necessary,however, that at least two of them are different from each other.

Moreover, in a case in which radiation having the fourth energy isemitted in addition to the radiation having the first to third energies,a fourth period T4 can be provided in addition to the first period T1,the second period T2, and the third period T3, and the radiation havingthe fourth energy can be emitted in the fourth period. In this case, ineach of the plurality of pixels 112, during a period that includes thefirst period T1, the second period T2, the third period T3, and thefourth period, the reset switch 220 (reset portion) does not reset theconversion element 210. The first to fourth energies can be differentfrom each other. It is only necessary, however, that at least two ofthem are different from each other.

The third to sixth modes will be described below more specifically. FIG.7 shows the operation of the radiation imaging apparatus 100 orradiation imaging system 1 in the third mode. In FIG. 7, the abscissaindicates a time. “Radiation energy” is energy of radiation which isemitted from the radiation source 400 and irradiates the radiationimaging apparatus 100. “PRES” is the reset signal PRES. “TS1” is thesample and hold signal TS1. “DOUT” is an output of the A/D convertor170. The control apparatus 350 can control synchronization of radiationemission from the radiation source 400 and the operation of theradiation imaging apparatus 100. The timing generator 130 controls anoperation in the radiation imaging apparatus 100. The clamp signal PCLis also activated over a predetermined period in a period during whichthe reset signal PRES is activated, and the clamp circuit 260 clamps anoise level.

The conversion element 210 is reset by activating the reset signal PRESover a predetermined period, and then the radiation 511 having firstenergy E1 and the radiation 512 having second energy E2 different fromthe first energy E1 are emitted. Note that the radiation 511 and theradiation 512 may be emitted successively in terms of time or with atime interval between them.

The irradiation times of the radiations 511 and 512 are preset, and asample and hold operation performed by the sample and hold circuit 280in accordance with the sample and hold signal TS1 ends immediatelybefore irradiation with the radiation 512. Note that in accordance withthe sample and hold signal TS1, the sample and hold circuit 280 samplesand holds a signal generated by irradiation with the radiation 511having the first energy E1.

Unlike the second mode, reset according to the end of a sample and holdoperation in the first period T1 is not performed in the third mode. Inother words, in the third mode, reset is not performed in the periodthat includes the first period T1 and the second period T2. Therefore,charges (electrical signal) generated by irradiation with the radiation511 of the first energy E1 remain in the charge accumulation portion ofthe conversion element 210.

Signals sampled and held by the sample and hold circuit 280 inaccordance with the sample and hold signal TS1 are output, from theradiation imaging apparatus 100, as the radiation signals 513corresponding to irradiation with the radiation 511 of the first energyE1.

Subsequently to irradiation with the radiation 511 of the first energyE1 in the first period T1, irradiation with the radiation 512 of thesecond energy E2 is performed in the second period T2. Consequently, inaddition to charges generated by irradiation with the radiation of thefirst energy E1 in the first period T1, charges generated by irradiationwith the radiation of the second energy E2 in the second period T2 areaccumulated in the charge accumulation portion of the conversion element210. The clamp circuit 260 outputs a radiation signal corresponding tothe charges accumulated in the conversion element 210.

When the output of the radiation signals 513 ends, the sample and holdcircuit 280 performs a sample and hold operation in accordance with thesample and hold signal TS1. Consequently, the sample and hold circuit280 samples and holds radiation signals corresponding to the chargesgenerated by irradiation with the radiation 511 of the first energy E1in the first period T1 and charges generated by irradiation with theradiation 512 of the second energy E2 in the second period T2.Subsequently, the signals sampled and held by the sample and holdcircuit 280 are output as radiation signals 515 from the radiationimaging apparatus 100.

The signal processor 352 of the control apparatus 350 obtains asubtraction image by processing the radiation signals 513 and theradiation signals 515 in accordance with the energy subtraction method.Note that the signal processor 352 can obtain, by subtracting the valueof each radiation signal 513 from the value of a corresponding one ofthe radiation signals 515, a radiation image generated by irradiationwith the radiation 512 of the second energy E2. That is, as in the firstmode and second mode, a radiation image generated by irradiation withthe radiation of the first energy and a radiation image generated byirradiation with the radiation of the second energy can also be obtainedin the third mode. The subtraction image can be obtained by processingthese radiation images in accordance with the energy subtraction method.

In the third mode, reset is not performed in the period that includesthe first period T1 and the second period T2, and thus the radiationinformation is never lost by reset. Furthermore, in the third mode,irradiation with the radiation 512 of the second energy is startedbefore the output of the radiation signals 513 ends as in the secondmode. Therefore, the third mode is superior to the first mode inradiation imaging of the fast-moving object.

Note that in the third mode, a radiation signal corresponding to the sumof the charges generated by the radiation 511 of the first energy E1 andthe charges generated by irradiation with the radiation 512 of thesecond energy E2 needs to be read out. As described above, in thearrangement shown in FIG. 4, the potential of the charge accumulationportion of the conversion element may be changed by charge injectionwhen the charges generated by the radiation 511 of the first energy E1are read out, destructing some signals. It is therefore preferable, inorder to execute the third mode, to adopt a pixel capable ofnondestructively reading out charges (signal) generated in thephotoelectric convertor (charge accumulation portion) as in thearrangement shown in FIG. 3.

Even with the arrangement shown in FIG. 3, the potential of the chargeaccumulation portion of the conversion element may be changed by chargedistribution in driving to change sensitivity, destructing some signals.It is therefore preferable, in order to execute the third mode, to adoptdriving not to change sensitivity. From the above, it can be said thatthe charges of the charge accumulation portion are not preferablydestructed in order to execute the third mode. More specifically, thearrangement, as exemplified in FIG. 3, with one or more transistors eachhaving the first main electrode which is connected to the chargeaccumulation portion, the second main electrode which is not connectedto the charge accumulation portion, and a control electrode will beconsidered. In such an arrangement, it is preferable that a voltageapplied to the control electrode of the one or more transistor is notchanged during the period that includes the first period T1 and thesecond period T2. However, in an application capable of allowingdestruction of some signals, driving to change sensitivity can also beadopted in the arrangements shown in FIGS. 3 and 4.

As described above, the clamp signal PCL can also be activated over thepredetermined period in the period during which the reset signal PRES isactivated, and the clamp circuit 260 clamps the noise level, and thenthe sample and hold circuit 270 can sample and hold this noise level. Inthe first to third modes, when a signal is read out from each pixel 112,a radiation signal can be read out from the sample and hold circuit 280(first signal holding portion), and a noise level can be read out fromthe sample and hold circuit 270 (second signal holding portion). Theamplifier unit 160 can perform differential amplification on a pair ofthe radiation signal and noise level thus read out. That is, adifference between the radiation signal and the noise level can beamplified.

In the fourth to sixth modes, the sample and hold circuits 270, 280, and290 are used to output radiation images of three or four energies to beseparable from each other. FIG. 8 shows the operation of the radiationimaging apparatus 100 or radiation imaging system 1 in the fourth mode.In FIG. 8, the abscissa indicates a time. “Radiation energy” is energyof radiation which is emitted from the radiation source 400 andirradiates the radiation imaging apparatus 100. “PRES” is the resetsignal PRES. “TS1” is the sample and hold signal TS1. “TS2” is thesample and hold signal TS2. “DOUT” is an output of the A/D convertor170. The control apparatus 350 can control synchronization of radiationemission from the radiation source 400 and the operation of theradiation imaging apparatus 100. The timing generator 130 controls anoperation in the radiation imaging apparatus 100. The clamp signal PCLis also activated over a predetermined period in a period during whichthe reset signal PRES is activated, and the clamp circuit 260 clamps anoise level.

The conversion element 210 is reset by activating the reset signal PRESover a predetermined period. Subsequently, radiation 601 having thefirst energy E1, radiation 602 of the second energy E2, and radiation603 having third energy E3 are emitted. The first to third energies E1to E3 can be different from each other. It is only necessary, however,that at least two of them are different from each other. Note that theradiations 601, 602, and 603 may be emitted successively in terms oftime or with a time interval between them. In the fourth mode, theconversion element 210 is not reset in a period that includes the firstperiod T1 during which the radiation 601 is emitted, the second periodT2 during which the radiation 602 is emitted, and the third period T3during which the radiation 603 is emitted.

The irradiation times of the radiations 601, 602, and 603 are preset andbefore irradiation with the radiation 601, the sample and hold signal TNis activated over a predetermined period after the reset signal PRES isactivated over the predetermined period. The conversion element 210 isreset by activating the reset signal PRES over the predetermined period.At this time, the clamp signal PCL is also activated over apredetermined period, and the clamp circuit 260 clamps a noise level.Then, the sample and hold circuit 270 can sample and hold the noiselevel by activating the sample and hold signal TN over a predeterminedperiod. This noise level is indicated as “F” in FIG. 8.

Next, the radiation 601 of the first energy E1 is emitted. Then, asample and hold operation performed by the sample and hold circuit 280in accordance with the sample and hold signal TS1 ends immediatelybefore the next irradiation with the radiation 602 of the second energyE2. Note that in accordance with the sample and hold signal TS1, thesample and hold circuit 280 samples and holds a signal (E1+F) which isobtained by adding the signal (E1) generated by irradiation with theradiation 511 having the first energy E1 to the noise level (F) of theclamp circuit 260. The signal (E1+F) sampled and held by the sample andhold circuit 280 in accordance with the sample and hold signal TS1 isoutput, from the radiation imaging apparatus 100, as the radiationsignal 513 corresponding to irradiation with the radiation of the firstenergy E1. At this time, the amplifier unit 160 performs differentialamplification on a radiation signal (S1=E1+F) sampled and held by thesample and hold circuit 280, and a noise level (N=F) sampled and held bythe sample and hold circuit 270. Accordingly, radiation signals 604 eachcorresponding to S1−N=(E1+F)−F=E1 are output from the radiation imagingapparatus 100.

Subsequently to irradiation with the radiation 601 of the first energyE1 in the first period T1, irradiation with the radiation 602 of thesecond energy E2 is performed in the second period T2. Consequently, inaddition to charges generated by irradiation with the radiation 601 ofthe first energy E1 in the first period T1, charges generated byirradiation with the radiation 602 of the second energy E2 in the secondperiod T2 are accumulated in the charge accumulation portion of theconversion element 210. The clamp circuit 260 outputs a radiation signalcorresponding to the charges accumulated in the conversion element 210.

Immediately before the next irradiation with the radiation 603 of thethird energy E3, a sample and hold operation performed by the sample andhold circuit 290 in accordance with the sample and hold signal TS2 ends.Note that in accordance with the sample and hold signal TS2, the sampleand hold circuit 290 samples and holds a signal (E1+E2+F) which isobtained by adding the signal (E2) generated by irradiation with theradiation 602 having the second energy E2 to the signal corresponding to(E1+F). This sampled and held signal (E1+E2+F) is output, from theradiation imaging apparatus 100, as radiation signals 605 correspondingto irradiation with the radiation 601 of the first energy E1 and theradiation 602 of the second energy E2. At this time, the amplifier unit160 performs differential amplification on a radiation signal(S2=E1+E2+F) sampled and held by the sample and hold circuit 290, andthe noise level (N=F) sampled and held by the sample and hold circuit270. Accordingly, the radiation signals 605 each corresponding toS2−N=(E1+E2+F)−F=E1+E2 are output from the radiation imaging apparatus100. Note that a signal via the column signal line 321 can be suppliedto one of the differential input pair of the amplifier unit 160, and asignal selected out of signals via the column signal line 322 and columnsignal line 323 can be supplied to the other of the differential inputpair.

Subsequently, after the end of irradiation with the radiation 603 of thethird energy E3, the sample and hold circuit 280 performs a sample andhold operation in accordance with the sample and hold signal TS1. Notethat in accordance with the sample and hold signal TS1, the sample andhold circuit 280 samples and holds a signal (E1+E2+E3+F) which isobtained by adding the signal (E3) generated by irradiation with theradiation 603 having the third energy E3 to the signal corresponding to(E1+E2+F). This sampled and held signal (E1+E2+E3+F) is output, from theradiation imaging apparatus 100, as a radiation signal 606 correspondingto irradiation with the radiations 601 to 603 of the first to thirdenergies E1 to E3. At this time, the amplifier unit 160 performsdifferential amplification on a radiation signal (S1=E1+E2+E3+F) sampledand held by the sample and hold circuit 280, and the noise level (N=F)sampled and held by the sample and hold circuit 270. Accordingly, theradiation signal 606 corresponding to S1−N=(E1+E2+E3+F)−F=E1+E2+E3 isoutput from the radiation imaging apparatus 100.

The signal processor 352 of the control apparatus 350 obtains asubtraction image by processing the radiation signals 604, 605, and 606in accordance with the energy subtraction method. Note that the signalprocessor 352 can obtain, by subtracting the value of each radiationsignal 605 from the value of the radiation signal 606, a radiation imagegenerated by irradiation with the radiation 603 of the third energy E3.The signal processor 352 can also obtain, by subtracting the value ofeach radiation signal 604 from the value of a corresponding one of theradiation signals 605, a radiation image generated by irradiation withthe radiation 602 of the second energy E2. Thus, the signal processor352 can obtain the radiation images of the first, second, and thirdenergies E1, E2, and E3. The subtraction image can be obtained byprocessing these radiation images in accordance with the energysubtraction method.

FIG. 9 shows the operation of the radiation imaging apparatus 100 orradiation imaging system 1 in the fifth mode. In FIG. 9, the abscissaindicates a time. “Radiation energy” is energy of radiation which isemitted from the radiation source 400 and irradiates the radiationimaging apparatus 100. “PRES” is the reset signal PRES. “TS1” is thesample and hold signal TS1. “TS2” is the sample and hold signal TS2.“DOUT” is an output of the A/D convertor 170. The control apparatus 350can control synchronization of radiation emission from the radiationsource 400 and the operation of the radiation imaging apparatus 100. Thetiming generator 130 controls an operation in the radiation imagingapparatus 100. The clamp signal PCL is also activated over apredetermined period in a period during which the reset signal PRES isactivated, and the clamp circuit 260 clamps a noise level.

The conversion element 210 is reset by activating the reset signal PRESover a predetermined period. Subsequently, radiation 701 having thefirst energy E1, radiation 702 having the second energy E2, radiation703 having the third energy E3, and radiation 704 having fourth energyE4 are emitted. The first to fourth energies E1 to E4 can be differentfrom each other. It is only necessary, however, that at least two ofthem are different from each other. Note that the radiations 701, 702,703, and 704 may be emitted successively in terms of time or with a timeinterval between them. In the fifth mode, the conversion element 210 isnot reset in a period that includes the first period T1 during which theradiation 701 is emitted, the second period T2 during which theradiation 702 is emitted, the third period T3 during which the radiation703 is emitted, and the fourth period T4 during which the radiation 704is emitted.

The irradiation times of the radiations 701 to 704 are preset and beforeirradiation with the radiation 701, the reset signal PRES is activatedover a predetermined period (not shown). The conversion element 210 isreset by activating the reset signal PRES over the predetermined period.

First, the radiation 701 of the first energy E1 is emitted. Then, asample and hold operation performed by the sample and hold circuit 270in accordance with the sample and hold signal TN ends immediately beforethe next irradiation with the radiation 702 of the second energy E2.Note that in accordance with the sample and hold signal TN, the sampleand hold circuit 270 samples and holds the signal (E1+F) which isobtained by adding the signal (E1) generated by irradiation with theradiation 701 having the first energy E1 to the noise level (F) of theclamp circuit 260.

Immediately before the next irradiation with the radiation 702 of thesecond energy E2, a sample and hold operation performed by the sampleand hold circuit 280 in accordance with the sample and hold signal TS1ends. Note that in accordance with the sample and hold signal TS1, thesample and hold circuit 280 samples and holds the signal (E1+E2+F) whichis obtained by adding the signal (E2) generated by irradiation with theradiation 702 having the second energy E2 to the signal corresponding to(E1+F).

Subsequently, the amplifier unit 160 performs differential amplificationon a radiation signal (S1=E1+E2+F) sampled and held by the sample andhold circuit 280, and the noise level (N=F) sampled and held by thesample and hold circuit 270. Then, radiation signals 705 eachcorresponding to S1−N=(E1+E2+F)−(E1+F)=E2 are output from the radiationimaging apparatus 100.

Immediately before the next irradiation with the radiation 703 of thethird energy E3, a sample and hold operation performed by the sample andhold circuit 290 in accordance with the sample and hold signal TS2 ends.Note that in accordance with the sample and hold signal TS2, the sampleand hold circuit 290 samples and holds the signal (E1+E2+E3+F) which isobtained by adding the signal (E3) generated by irradiation with theradiation 703 having the third energy E3 to the signal corresponding to(E1+E2+F).

Subsequently, the amplifier unit 160 performs differential amplificationon the radiation signal (S1=E1+E2+E3+F) sampled and held by the sampleand hold circuit 290, and the noise level (N=F) sampled and held by thesample and hold circuit 270. Then, a radiation signal 706 correspondingto S2−N=(E1+E2+E3+F)−(E1+F)=E2+E3 is output from the radiation imagingapparatus 100.

Furthermore, after irradiation with the radiation 704 of the fourthenergy E4, the sample and hold circuit 280 performs a sample and holdoperation in accordance with the sample and hold signal TS1. Note thatin accordance with the sample and hold signal TS1, the sample and holdcircuit 280 samples and holds a signal (E1+E2+E3+E4+F) which is obtainedby adding the signal (E4) generated by irradiation with the radiation704 having the fourth energy E4 to the signal corresponding to(E1+E2+E3+F).

Subsequently, the sample and hold signal TN is activated over apredetermined period after the reset signal PRES is activated over thepredetermined period. The conversion element 210 is reset by activatingthe reset signal PRES over the predetermined period. At this time, theclamp signal PCL is also activated over a predetermined period, and theclamp circuit 260 clamps a noise level. Then, the sample and holdcircuit 270 can sample and hold the noise level (F) by activating thesample and hold signal TN over a predetermined period.

Subsequently, the amplifier unit 160 performs differential amplificationon a radiation signal (S1=E1+E2+E3+E4+F) sampled and held by the sampleand hold circuit 280, and the noise level (N=F) sampled and held by thesample and hold circuit 270. Then, the radiation signal 706corresponding to S1−N=(E1+E2+E3+E4+F)−(E1+F)=E2+E3+E4 is output from theradiation imaging apparatus 100.

Subsequently, the amplifier unit 160 performs differential amplificationon the radiation signal (S1=E1+E2+E3+E4+F) sampled and held by thesample and hold circuit 270, and the noise level (N=F) sampled and heldby the sample and hold circuit 270. Then, the radiation signal 706corresponding to S1−N=(E1+E2+E3+E4+F)−(F)=E1+E2+E3+E4 is output from theradiation imaging apparatus 100.

The signal processor 352 of the control apparatus 350 obtains asubtraction image by processing the radiations 701, 702, 703, and 704 inaccordance with the energy subtraction method. Note that the signalprocessor 352 can obtain, by subtracting the value of a radiation signal707 from the value of a radiation signal 708, a radiation imagegenerated by irradiation with the radiation 704 of the fourth energy E4.The signal processor 352 can also obtain, by subtracting the value ofthe radiation signal 706 from the value of the radiation signal 707, aradiation image generated by irradiation with the radiation 701 of thefirst energy E1. The signal processor 352 can further obtain, bysubtracting the value of each radiation signal 705 from the value of theradiation signal 706, a radiation image generated by irradiation withthe radiation 703 of the third energy E3.

Thus, the signal processor 352 can obtain the radiation images of thefirst, second, third, and fourth energies E1, E2, E3, and E4. Thesubtraction image can be obtained by processing these radiation imagesin accordance with the energy subtraction method.

It is further possible to obtain radiation images of more energies byincreasing the number of sample and hold portions.

The second to fifth modes are suitable for a case in which the radiationsource 400 capable of changing radiation energy at a high speed isavailable. The radiation energy can be changed stepwise as in theabove-described examples but may be changed successively. The radiationenergy can be changed by changing the tube voltage of the radiationsource 400. Alternatively, the radiation energy may be changed byemitting radiation having a wide energy band (wavelength band) from aradiation source and switching a plurality of filters.

FIG. 10 shows the operation of the radiation imaging apparatus 100 orradiation imaging system 1 in the sixth mode. In FIG. 10, the abscissaindicates a time. “Radiation energy” is energy of radiation which isemitted from the radiation source 400 and irradiates the radiationimaging apparatus 100. “PRES” is the reset signal PRES. “TS1” is thesample and hold signal TS1. “TS2” is the sample and hold signal TS2.“DOUT” is an output of the A/D convertor 170. The control apparatus 350can control synchronization of radiation emission from the radiationsource 400 and the operation of the radiation imaging apparatus 100. Thetiming generator 130 controls an operation in the radiation imagingapparatus 100. The clamp signal PCL is also activated over apredetermined period in a period during which the reset signal PRES isactivated, and the clamp circuit 260 clamps a noise level.

In the sixth mode, the fact that the waveform (waveform change) of theradiation energy generated by the radiation source 400 is notrectangular is used. As exemplified in FIG. 10, rising and falling ofradiation may not be rectangular. Note that the waveform that is notrectangular may be formed on purpose. In FIG. 10, the waveform ofradiation 800 includes radiations 801, 802, and 803. The average valueE1 of the energy of the radiation 801 in the period T1, the averagevalue E2 of the energy of the radiation 802 in the period T2, and theaverage value E3 of the energy of the radiation in the period T3 aredifferent from each other. The energy subtraction method can beimplemented by using this.

Before irradiation with the radiation 800, the reset signal PRES isactivated over a predetermined period, and then the sample and holdsignal TN is activated over a predetermined period. The conversionelement 210 is reset by activating the reset signal PRES over thepredetermined period. At this time, the clamp signal PCL is alsoactivated over a predetermined period, and the clamp circuit 260 clampsa noise level. Then, the sample and hold circuit 270 can sample and holdthe noise level by activating the sample and hold signal TN over thepredetermined period.

Then, irradiation with the radiation 800 is started. Immediately beforethe period T2, a sample and hold operation performed by the sample andhold circuit 280 in accordance with the sample and hold signal TS1 ends.Note that in accordance with the sample and hold signal TS1, the sampleand hold circuit 280 samples and holds the signal (E1+F) which isobtained by adding the signal (E1) generated by irradiation with theradiation 511 having the first energy E1 to the noise level (F) of theclamp circuit 260. The signal (E1+F) sampled and held by the sample andhold circuit 280 in accordance with the sample and hold signal TS1 isoutput, from the radiation imaging apparatus 100, as radiation signals804 corresponding to irradiation with the radiation of the first energyE1. At this time, the amplifier unit 160 performs differentialamplification on a radiation signal (S1=E1+F) sampled and held by thesample and hold circuit 280, and the noise level (N=F) sampled and heldby the sample and hold circuit 270. Accordingly, the radiation signals804 each corresponding to S1−N=(E1+F)−F=E1 are output from the radiationimaging apparatus 100.

In the period T2, in addition to charges generated by irradiation withthe radiation 801 of the first energy E1 in the first period T1, chargesgenerated by irradiation with the radiation 802 of the second energy E2in the second period T2 are accumulated in the charge accumulationportion of the conversion element 210. The clamp circuit 260 outputs aradiation signal according to the charges accumulated in the conversionelement 210.

Immediately before the period T3, a sample and hold operation performedby the sample and hold circuit 290 in accordance with the sample andhold signal TS2 ends. Note that in accordance with the sample and holdsignal TS2, the sample and hold circuit 290 samples and holds the signal(E1+E2+F) which is obtained by adding the signal (E2) generated byirradiation with the radiation 802 having the second energy E2 to thesignal corresponding to (E1+F). This signal (E1+E2+F) is output, fromthe radiation imaging apparatus 100, as radiation signals 805corresponding to the radiation 801 of the first energy E1 and theradiation 802 of the second energy E2. At this time, the amplifier unit160 performs differential amplification on the radiation signal(S2=E1+E2+F) sampled and held by the sample and hold circuit 290, andthe noise level (N=F) sampled and held by the sample and hold circuit270. Accordingly, the radiation signals 805 each corresponding toS2−N=(E1+E2+F)−F=E1+E2 are output from the radiation imaging apparatus100.

Subsequently, the sample and hold circuit 280 performs a sample and holdoperation in accordance with the sample and hold signal TS1 after theend of the period T3 and the end of the radiation signals 804. Note thatin accordance with the sample and hold signal TS1, the sample and holdcircuit 280 samples and holds the signal (E1+E2+E3+F) which is obtainedby adding the signal (E3) generated by irradiation with the radiation803 having the third energy E3 to the signal corresponding to (E1+E2+F).This sampled and held signal (E1+E2+E3+F) is output, from the radiationimaging apparatus 100, as a radiation signal 806 corresponding toirradiation with the radiations 801 to 803 of the first to thirdenergies E1 to E3. At this time, the amplifier unit 160 performsdifferential amplification on the radiation signal (S1=E1+E2+E3+F)sampled and held by the sample and hold circuit 280, and the noise level(N=F) sampled and held by the sample and hold circuit 270. Accordingly,the radiation signal 806 corresponding to S1−N=(E1+E2+E3+F)−F=E1+E2+E3is output from the radiation imaging apparatus 100.

The signal processor 352 of the control apparatus 350 obtains asubtraction image by processing the radiation signals 804, 805, and 806in accordance with the energy subtraction method. Note that the signalprocessor 352 can obtain, by subtracting the value of each radiationsignal 805 from the value of the radiation signal 806, a radiation imagegenerated by irradiation with the radiation 803 of the third energy E3.The signal processor 352 can also obtain, by subtracting the value ofeach radiation signal 804 from the value of a corresponding one of theradiation signals 805, a radiation image generated by irradiation withthe radiation 802 of the second energy E2. Thus, the signal processor352 can obtain the radiation images of the first, second, and thirdenergies E1, E2, and E3. The signal processor 352 can obtain thesubtraction image by processing these radiation images in accordancewith the energy subtraction method.

Other Embodiments

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2016-219952, filed Nov. 10, 2016, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A radiation imaging apparatus that obtains a radiation image by an energy subtraction method of obtaining a new image by processing a plurality of images obtained by capturing an object a plurality of times while changing energy of radiation to irradiate the object, the apparatus comprising: a pixel array that includes a plurality of pixels, wherein each of the plurality of pixels includes a conversion element that converts radiation into an electrical signal and a reset portion that resets the conversion element, each of the plurality of pixels performs an operation of outputting a first signal corresponding to an electrical signal generated by the conversion element in a first period, and an operation of outputting a second signal corresponding to an electrical signal generated by the conversion element in the first period and a second period different from the first period, radiation having first energy is emitted in the first period, and radiation having second energy is emitted in the second period, and the radiation imaging apparatus has a mode in which, in each of the plurality of pixels, the reset portion does not reset the conversion element during a period that includes the first period and the second period.
 2. The apparatus according to claim 1, wherein each of the conversion elements includes a charge accumulation portion that accumulates charges corresponding to radiation as an electrical signal, each of the plurality of pixels includes at least one transistor having a first main electrode which is connected to the charge accumulation portion, a second main electrode which is not connected to the charge accumulation portion, and a control electrode, and the at least one transistor electrically connects the first main electrode and the second main electrode by receiving an ON voltage at the control electrode, and in the mode, a voltage applied to the control electrode of the at least one transistor does not change during the period that includes the first period and the second period.
 3. The apparatus according to claim 1, wherein the first period is started after each of the reset portions resets the conversion element.
 4. The apparatus according to claim 1, wherein each of the plurality of pixels further includes a holding portion that holds a signal corresponding to an electrical signal generated by the conversion element, and each of the plurality of pixels outputs the first signal and the second signal via the holding portion.
 5. The apparatus according to claim 1, wherein each of the plurality of pixels further includes a first holding portion that holds the first signal and a second holding portion that holds the second signal, and each of the plurality of pixels outputs the first signal via the first holding portion and the second signal via the second holding portion.
 6. The apparatus according to claim 1, wherein each of the plurality of pixels includes a plurality of holding portions that hold signals corresponding to electrical signals generated by the conversion element, and each of the plurality of pixels outputs the first signal and the second signal via one of the plurality of holding portions.
 7. The apparatus according to claim 1, wherein each of the plurality of pixels includes a first signal holding portion that holds a signal corresponding to an electrical signal generated by the conversion element and a second signal holding portion that holds a signal corresponding to a noise level, and the radiation imaging apparatus further comprises a readout circuit that reads out, from each of the plurality of pixels, a pair of the signal held by the first signal holding portion and the signal held by the second signal holding portion, and an amplifier unit that amplifies a difference between the pair of signals read out by the readout circuit.
 8. The apparatus according to claim 1, wherein each of the plurality of pixels further performs, in a third period different from the first period and the second period, an operation of outputting third signals corresponding to electrical signals generated by the conversion element in the first period and the second period, radiation having third energy is emitted in the third period, and in each of the plurality of pixels, the reset portion does not rest the conversion element in a period that includes the first period, the second period, and the third period.
 9. The apparatus according to claim 8, wherein each of the plurality of pixels includes a plurality of holding portions that hold signals corresponding to electrical signals generated by the conversion element, and each of the plurality of pixels outputs the first signal, the second signal, and the third signal via one of the plurality of holding portions.
 10. The apparatus according to claim 8, wherein each of the plurality of pixels includes a plurality of first signal holding portions that hold signals corresponding to electrical signals generated by the conversion element and a second signal holding portion that holds a signal corresponding to an electrical signal generated by the conversion element, and the radiation imaging apparatus further comprises a readout circuit that reads out, from each of the plurality of pixels, a pair of a signal which is held by the first signal holding portion selected out of the plurality of first signal holding portions and a signal which is held by the second signal holding portion, and an amplifier unit that amplifies a difference between the pair of signals read out by the readout circuit.
 11. The apparatus according to claim 1, further comprising a signal processor that generates a radiation image by the energy subtraction method based on the first signal and the second signal.
 12. A radiation imaging system comprising: a radiation imaging apparatus defined in claim 1; and a control apparatus that controls a radiation source and the radiation imaging apparatus.
 13. A radiation imaging system that obtains a radiation image by an energy subtraction method of obtaining a new image by processing a plurality of images obtained by capturing an object a plurality of times while changing energy of radiation to irradiate the object, the system comprising: a pixel array that includes a plurality of pixels; and a signal processor that processes a signal output from the pixel array, wherein each of the plurality of pixels includes a conversion element that converts radiation into an electrical signal and a reset portion that resets the conversion element, each of the plurality of pixels performs an operation of outputting a first signal corresponding to an electrical signal generated by the conversion element in a first period, and an operation of outputting a second signal corresponding to an electrical signal generated by the conversion element in the first period and a second period different from the first period, radiation having first energy is emitted in the first period, and radiation having second energy is emitted in the second period, the radiation imaging system has a mode in which, in each of the plurality of pixels, the reset portion does not reset the conversion element during a period that includes the first period and the second period, and the signal processor generates a radiation image by the energy subtraction method based on the first signal and the second signal.
 14. The system according to claim 13, wherein each of the conversion elements includes a charge accumulation portion that accumulates charges corresponding to radiation as an electrical signal, each of the plurality of pixels includes at least one transistor having a first main electrode which is connected to the charge accumulation portion, a second main electrode which is not connected to the charge accumulation portion, and a control electrode, and the at least one transistor electrically connects the first main electrode and the second main electrode by receiving an ON voltage at the control electrode, and in the mode, a voltage applied to the control electrode of the at least one transistor does not change during the period that includes the first period and the second period.
 15. The system according to claim 13, further comprising a controller that controls a radiation source to send a command to emit the radiation having the first energy in the first period and emit the radiation having the second energy in the second period.
 16. The system according to claim 13, further comprising a controller that controls the pixel array such that a period during which radiation emitted from a radiation source has the first energy becomes the first period, and a period during which radiation emitted from the radiation source has the second energy becomes the second period.
 17. A radiation imaging method of obtaining a radiation image by an energy subtraction method using a radiation imaging apparatus, the energy subtraction method being a method of obtaining a new image by processing a plurality of images obtained by capturing an object a plurality of times while changing energy of radiation to irradiate the object, the radiation imaging apparatus including a pixel array that includes a plurality of pixels, and each of the plurality of pixels including a conversion element that converts radiation into an electrical signal and a reset portion that resets the conversion element, the method comprising: causing each of the plurality of pixels to perform an operation of outputting a first signal corresponding to an electrical signal generated by the conversion element in a first period, and an operation of outputting a second signal corresponding to an electrical signal generated by the conversion element in the first period and a second period different from the first period, and obtaining a radiation image based on a signal corresponding to the first signal and a signal corresponding to the second signal, wherein radiation having first energy is emitted in the first period, and radiation having second energy is emitted in the second period, and in the causing and the obtaining, in each of the plurality of pixels, the reset portion does not reset the conversion element during a period that includes the first period and the second period.
 18. The method according to claim 17, wherein each of the conversion elements includes a charge accumulation portion that accumulates charges corresponding to radiation as an electrical signal, each of the plurality of pixels includes at least one transistor having a first main electrode which is connected to the charge accumulation portion, a second main electrode which is not connected to the charge accumulation portion, and a control electrode, and the at least one transistor electrically connects the first main electrode and the second main electrode by receiving an ON voltage at the control electrode, and a voltage applied to the control electrode of the at least one transistor is not changed during the period that includes the first period and the second period. 